He was a deputy circuit clerk of Buchanan County, Missouri from 1911 to 1917, and read law to be admitted to the bar in 1916, entering private practice in St. Joseph from 1917 to 1926.
[1][2] Duncan served as delegate to the 1932 Democratic National Convention.
He was elected as a Democrat to the United States House of Representatives of the 73rd United States Congress and to the four succeeding Congresses, serving from March 4, 1933 to January 3, 1943.
He served as Chairman of the Democratic Caucus for the 77th United States Congress, but was unsuccessful in his candidacy for reelection in 1942 to the 78th United States Congress.
He served as Chief Judge of the Western District from 1954 to 1959, assuming senior status on May 31, 1965, and serving in that capacity until his death on August 1, 1974, in Kansas City, Missouri, where he resided.